In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Industrial use of PVD TiB2 layers has so far been strongly limited due to the very high compressive stresses normally possessed by such layers. In recent years large efforts have been made to develop PVD processes for deposition of new ultra-hard thin layers, e.g. boron carbide (B4C), titanium diboride (TiB2) and cubic boron nitride (c-BN), for wear protection, especially of cutting tools. Although these layer materials are very attractive for the tooling industry, they have not yet become commercialized.
The high hardness and Young's modulus of TiB2, as well as its chemical resistance, are attributed to the crystal structure and atomic bonding. In TiB2 the Ti atoms form a metallic hexagonal structure. In analogy with the usual notation ABABAB for hexagonal close packing, the stacking sequence of Ti in TiB2 will be AAA. The boron (B) atoms are situated interstitially between the A-layers forming a strong covalently bonded hexagonal net. The sequence may be described as AHAHAH where H denotes a boron layer. The combination of metallic Ti and strongly covalently bonded B results in a compound with high thermal and electrical conductivity as well as high yield strength and chemical resistance.
TiB2 layers have been deposited by various PVD techniques, such as reactive sputtering, arc evaporation, and most commonly, magnetron sputtering. However, despite the very interesting properties of the TiB2 bulk material, these layers are generally of little commercial interest. Their stress level is too high, which limits the practical adhesion, and thereby the layer thickness. In addition, due to the high intrinsic stressed layers are too brittle and easily fail because of lack of cohesion.
U.S. Pat. No. 4,019,873 discloses a coated cemented carbide cutting tool insert. The coating is composed of two superimposed layers including an outer, extremely wear-resistant layer consisting essentially of aluminium oxide and/or zirconium oxide. The inner layer is composed of at least one boride selected from the group consisting of borides of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten.
U.S. Pat. No. 4,268,582 discloses a coated cemented carbide cutting tool insert comprising a cemented carbide substrate, the surface regions thereof having diffused therein an element such as boron, silicon or aluminium. The inserts further comprise a layer disposed on the diffused substrate, the layer being a boride such as titanium boride, hafnium boride, zirconium boride or tantalum boride. In another embodiment, the coated cemented article further includes an interlayer sandwiched between the diffused substrate and the boride layer.
Both these patents deposit the TiB2-layers by CVD. However, because of the high temperature during CVD-deposition undesirable cobalt/boride-phases are formed. For that reason PVD deposition of TiB2-layers on WC-Co based substrates is preferred.
M. Berger, M. Larsson and S. Hogmark, Surf. Coat. Technol., 124 (2000) 253-261 have grown TiB2 layers with magnetron sputtering using negative substrate bias varying from −220V to −50 V, the residual stress were very high (i.e.—compressive stress from −10.2 GPa to −7.9 GPa). Also, one variant using 0 V was grown also resulting in a very high compressive residual stress of −6.1 GPa. In this investigation no films were grown using positive bias.
C. Mitterer, M. Rauter and P. Rodhamrnmer, Surf. Coat. Technol., 41 (1990) 351-363 have grown TiB2 and Ti—B—N—C compound layers with magnetron sputtering using negative substrate bias. The TiB2 layers were in high compressive residual stresses (˜−4 GPa).
R. Wiedemann and H. Oettel, Surface Engineering, 14 4 (1998) 299-304 also have grown TiB2 layers using magnetron sputtering. A negative substrate bias was used resulting in intrinsic compressive stress of ˜−2 GPa. The hardness was low (25-29 GPa).
One possibility to obtain a low compressive residual stress state boride layer would be, for example, to grow at a high pressure, or other condition reducing ion bombardment. However, this will give a layer with a columnar structure, often associated with grain boundaries that lack density, and a cauliflower shaped surface morphology. This microstructure is not preferable since a typical application of TiB2 layers are, for example, machining of soft and sticky aluminium alloys, which require the coating of the cutting tool to have a very smooth top surface in order to reduce the tendency to form a build-up edge. Of course, grain boundaries that are not dense are detrimental for the wear-resistance of the coating.